1. Field of the Invention
The present invention relates to a light emitting diode (LED) capable of enhancing light extraction efficiency and a process for fabricating the same.
2. Description of Related Art
A light emitting diode (LED) is a semiconductor device constituted mainly by group III-V compound semiconductor materials, for example. Since such semiconductor materials have a characteristic of converting electricity into light, when a current is applied to the semiconductor materials, electrons therein would be combined with holes and release excessive energy in a form of light, thereby achieving an effect of luminosity.
FIG. 1A is a schematic cross-sectional view of a conventional LED. Referring to FIG. 1A, a conventional LED 10 is generally constituted by a substrate 100, an n-type semiconductor layer 102 thereon, an active emitting layer 104, and a p-type semiconductor layer 106. Moreover, there is an n-type electrode 108 on a surface of the n-type semiconductor layer 102, and there is a p-type electrode 110 on a surface of the p-type semiconductor layer 106.
However, as to the conventional GaN LED, sapphire is used as an epitaxy substrate generally. Since sapphire is a transparent material, an LED fabricated by using sapphire would scatter light to all directions without focusing the light and thus cause consumption, as illustrated by arrows pointing below the substrate 100 from the active emitting layer 104 in FIG. 1A. Meanwhile, the scattered light would be absorbed by each semiconductor layer inside the LED and reserved in a form of heat. Therefore, emitting luminosity and efficiency of the GaN LED is lowered.
In order to enhance the luminosity efficiency of the LED, in a method recently disclosed by U.S. Pat. No. 6,091,085 or US 2003/057444 A1, an epitaxy sapphire substrate is fabricated as a periodic structure so as to reduce epitaxy defects in GaN and enhance its luminosity. Simultaneously, a one-dimensional periodic light grid structure may allow a portion of back light to be refracted so as to emit forward and be reused and thereby increasing an light extraction efficiency of the LED as a whole.
Nevertheless, the light grid structure does not refract vertical incident light well. Only an incident light with a larger incident angle may have an obvious reflection effect. Hence, the light grid structure achieves only limited results in recycling and reusing vertically-emitted light.
Recently, in U.S. Pat. No. 6,563,141, a distributed Bragg reflector (DBR) mirror has been disclosed, wherein the mirror is a planar multi-layer dielectric having alternate layers of dielectric material with a high refractive index ratio between the adjacent layers and a reflectivity of the order of 97% or more.
However, high temperature form epitaxial process will cause the following two phenomena. One is the break of the DBR mirror caused by thermal stress deformation because the DBR mirror and the substrate have different coefficient of thermal expansion; the other is the peeling between the DBR mirror and the substrate caused by the elastic deformation resulted from high temperature process. Therefore, the DBR mirror disclosed in U.S. Pat. No. 6,563,141 may peel off and be cracked after high temperature epitaxial process, and thus it is difficult to be commercialized, as shown in FIG. 1B. FIG. 1B is a scanning electron microscope (SEM) photograph of a top view of the planar multi-layer dielectric after annealing at 1100° C.